The present invention relates to communications systems, and in particular communications systems for use in multi-path, multi-user environments, such as cellular mobile communications systems.
In a cellular mobile communications system, the area to be served by the system is divided into a plurality of cells, each of which is served by a base station. Within each cell, it is necessary for the base station to transmit to each wanted user (i.e. each active mobile unit) in a multi-user and multi-path environment. In order to achieve satisfactory signal detection at low bit error rates, the multi-user interference (MUI), sometimes referred to also as multiple access interference (MAI), must be reduced to an acceptable level.
Code division multiple access (CDMA) is a multiplexing technique which is considered suitable for use in mobile cellular communications systems due to its spectral efficiency (leading to the accommodation of more users) and its relative immunity to interference. In cellular mobile communications systems employing CDMA, each signal to be transmitted from the base station to a mobile unit is spread over a wide bandwidth at the base station using a unique spreading code. Each mobile unit despreads one of the wide bandwidth signals back to the original signal using a replica of the spreading code that was used to spread that signal. Other wide bandwidth signals, which were spread with different spreading codes, are not despread, and thus these signals appear at the mobile unit only as background interference. The spreading codes are generally chosen to be orthogonal, or nearly orthogonal. In this way the transmitted signals themselves are made orthogonal, so that interference between the signals is minimised.
An advantage of CDMA is that signals transmitted to a mobile unit via different transmission paths can be resolved (i.e. detected independently) by the mobile unit, provided the signals have sufficiently large differential time delays. This can allow transmission signals transmitted via different paths to be detected separately at the mobile unit and then combined so as to increase the quality of the signals. However, differential time delays between transmission signals received by a mobile unit via different transmission paths can lead to interference between those signals. The increased interference may be due to signals transmitted to the same mobile unit via different transmission paths interfering with each other, or signals transmitted to different mobile units interfering with each other (MUI).
In an article by Y. J. Guo et al entitled “Advanced base station technologies for UTRAN”, Electronics & Communications Engineering Journal, June 2000, the entire content of which is incorporated herein by reference, it is proposed to employ adaptive antennas at base stations to form directional transmission beams for each user according to its location. Using an adaptive antenna, a relatively narrow transmission beam pointing at the wanted user and with nulls in the direction of strong interferers can be produced, so that MUI experienced by each user can be reduced.
The system proposed by Guo et al is effective in reducing the MUI experienced by each user. However, where two paths with a wide angular separation exist between the base station and a mobile unit, either the transmission beam must be broad enough to encompass both of the paths, or one of the paths must be excluded from the transmission beam, which may reduce the effectiveness of the system. Furthermore, the proposed system does not solve the problem of differential time delays leading to interference between the signals transmitted to the same user via the various paths.
It is therefore desirable to provide a system which overcomes the problems mentioned above. In particular it is desirable to provide a system in which interference between signals transmitted to a user via different transmission paths is reduced.